In most GC languages, the String type itself is not a managed handle to a single memory buffer, but a reference into a whole copy-on-write datastore. So you can't even scramble the underlying memory - mutating the string to erase it will not zap the original, it will just create a copy.

Your best bet is to ensure no references to the password string exist - including in library code you may use, which means constant revalidations whenever you update a library or your underlying language runtime. Once you do that, you can force garbage collection, either by some explicit language mechanism to request garbage collection, or by trying to allocate gobs of memory in some way that can't be easily optimized out.